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Monday, July 1, 2013

Let's brain-storm a bit on the Gulf Stream. This blog has come about because it has been reported that more of the Gulf Stream, which splits as it reaches it's northern limit , is flowing into the Arctic ocean through the Fram Strait than previously.

The Gulf Stream is the northern flowing, western branch of the North Atlantic Gyre. The Gyre consists of the Gulf Stream which flows from Florida to roughly the latitude of Newfoundland, the North Atlantic Drift which takes this water eastward towards Europe. the Norwegian branch that splits off and flows along the North West coast of Norway and the Southern Canary Current that flows southward along the coast of Europe and the Northern part of Africa. Finally there is Equatorial current that returns water along the Equator to North America in the region of the West Indies. Part of this Equatorial current enters the Gulf of Mexico, warms and leaves the Gulf in the region of Cuba to complete the circuit and become the Gulf Stream.

This gyre is primarily powered by wind. The Hadley cells which circulate air around the world, with the help of Coriolis result in Equatorial winds blowing westward from Africa to America and mid latitude winds in the latitude of Newfound land and the UK blowing eastward from Canada to Europe. These push the water resulting in the North Atlantic Gyre.

Even with a wind, water doesn't readily flow uphill so what could be sucking some of this reletively warm surface water into the semi enclosed Arctic ocean and what could be increasing this flow.

The obvious possibility is that somewhere water is leaving the Arctic ocean to be replaced by surface water sucked northward. Since the amount of water entering the Arctic Ocean is apparently increasing, the amount leaving should also be increasing.

First there is the possibility that evaporation is increasing. Evaporation, the transfer of H2O into the air from water, is far stronger than sublimation, the transfer of H2O into the air from ice. A molecule of water at 0 degrees C has far more energy than a molecule of ice at 0 degrees C not to mention water molecules which are above zero degrees. Year by year, there is more and more open water and so more water vapour should be entering the atmosphere.

Whether or not increased evaporation is increasing the exit of water from the Arctic ocean depends to some extent on where this water is falling. If it is falling on the catchment of the Arctic ocean and flows back immediately, the effect would be limited. If, however it falls as snow, even if it falls in the Arctic catchment, there would be a temporary exit of water until the snow melts and sends the water back to the ocean. If the water falls outside the Arctic Ocean catchment, there would be an overall effect.

A second effect is the freezing of sea water. As sea water freezes, it makes fresh water ice and the salt remains in the water below the ice. This increase in salinity plus the coldness of the water makes it heavier than the surrounding water and it flows down to the bottom and out through the Fram Straights. This outflow is balanced by a surface inflow. Why should this be increasing?

When the Arcic ocean is pretty well covered in ice at the September mimimum, to make more ice, heat has to conduct through the ice out into the air to created more ice at the bottom of the ice sheet. The thicker the ice, the greater the insulating effect. With a cover of snow on the ice, the insulation is even greater. Snow is full of air making it a very good insulator.

However if you subtract the minimum from the maximum ice volume, year by year, you find that, while the volume of ice which has formed each year,over the past few years has increased over the long term average, the increase is small compared to the total amount of ice formed. The average amount of ice formed each winter since 1979 is about 16.39km3 while over the past six years, starting in 2008, ice formation has been 18.69, 17.98, 16.51, 17.51, 17.91 and 18.56km3. This doesn't seem enough to explain much more warm surface water being sucked into the Arctic basin although 2008 and 2013 stand out as particularly high years.

The last effect I can think of is wind. If the wind is blowing more often from the South which with the added effect of Coriolis, will result in winds from the South West, this will push surface water toward the North East. In other words, it should increase the flow of the current that flows along the coast of Norway. This should tend to make it ice free along the coast of Russia more than was the case historically. So what could increase the incidence of winds from the south.

The jet stream, which is the marker between the Ferrel cell and the Polar Hadley cell has been creeping northward. In addition, the Rossby waves have been increasing in amplitude. Winds to the south of the Jet stream are westerlies so the further north the Jet stream, the further north the westerlies and hence a greater proportion of the Gulf Stream flowing up the coast of Norway.

I have long thought that the polar jet stream may close down and the Ferrel cell extend up into the Arctic. If this happens, we should have westerlies much further north and hence much more of the warm water of the Gulf Stream entering the Arctic Ocean Basin. This may explain the observations that in the past, temperate conditions extended right up to the shores of the Arctic ocean.